This invention is directed to a new and improved microetch formulation (composition) which is particularly suited for use in connection with the electroplating of through holes interconnecting the various metal layers of a multilayer circuit board.
Multiple layer copper clad circuit boards are used in constructing many electronic products. These circuit boards typically are copper cladding layers formed into various circuit configurations separated by dielectric material layers. In order to interconnect the copper layers, through holes are provided which extend between the layers. The through holes are electroplated with copper in order to form a conductive pathway between the copper clad layers. In order to facilitate electroplating of the through holes, a thin layer of the conductive polymer has been used see U.S. Pat. Nos. 5,528,000 and 5,415,762 (the entire contents of these patents are incorporated herein by reference hereto). The aforementioned patents should also be referred to for a description of the art. The composition herein is also usable in the direct plate processes such as palladium sulfide, palladium thiosulfate and carbon based direct plate processes (also see e.g., U.S. Pat. No. 4,810,333 which is incorporated herein by reference hereto for a direct plate process). In some processes, etchants have been used to remove the conductive coating, e.g. a conductive polymer positioned over the conductive copper cladding without removing the conductive polymer layer from the dielectric extending into the hole to expose the clad copper layers prior to electroplating. Such processes have used microetchant compositions which are based among other things on sulfuric acid and hydrogen peroxide combinations, see Printed Circuits Handbook, 4th Edition, Clyde Combs Edition McGraw-Hill .COPYRGT. 1996, Chapter 21, 4, Etching Solutions and in particular Chapter Section 21, 4,2 Sulfuric acid--Hydrogen Peroxide Etchant Systems (the entire contents of this chapter are incorporated herein by reference hereto). While this has helped, it has been found that the post bonding separation (interconnect defects) e.g. poor bonding of the deposited copper to the copper cladding, has to be reduced even further to enhance quality.
It has now been found that a new and improved microetch formulations of this invention and the method of use thereof will unexpectedly increase the capability of the etchant to remove the conductive polymer over the copper cladding leaving a surface which permits better bonding of the subsequently electroplated copper to the exposed copper cladding.
In this way, it is now possible to provide circuit boards having substantially fewer interconnect defects due to poor bonding between the copper cladding of the circuits extending to the hole side surface and the electroless deposited copper.